Aldolase-B-Gen (hereditäre Fruktoseintoleranz): the pathway in the body

Aldolase-B-Gen (hereditäre Fruktoseintoleranz) is part of the pathway “Fructose absorption”. This page shows the whole pathway; the station of Aldolase-B-Gen (hereditäre Fruktoseintoleranz) is highlighted.

Where this laboratory value sits: DHAP and glyceraldehyde — split by aldolase B. Aldolase B splits fructose-1-phosphate into dihydroxyacetone phosphate (DHAP) and glyceraldehyde. Its blueprint lies in the ALDOB gene; variants that alter the enzyme strongly leave fructose-1-phosphate lying in the cell. Source 8

In brief

Fructose is a simple sugar found in fruit and table sugar. The small intestine and liver absorb it via transporters and convert it into glucose and other substances; what reaches the large intestine is fermented by bacteria, producing hydrogen. There it binds water osmotically.

The pathway step by step

Each station states what the compound does there. Three signs: ↑ supplies — builds up or makes available · ↓ depletes — inhibits, consumes or withholds · ↕ both, depending on amount. Behind it stands what the statement rests on: established physiology, observed in studies, or contested. The signs do not grade; they name the direction.

1. Fructose in small bowel → Into the gut cell GLUT5 The transporter GLUT5 lets fructose into the cells of the small intestinal wall. It uses no energy and follows the gradient; its amount adjusts to the fructose supply. Once it is saturated, fructose stays in the gut. Source 1, 6↑ supplies GLUT5 is the bottleneck of absorption: it transports only along the concentration gradient and only as much as there are transporters in the gut wall. In animal models, fructose in the gut stimulates the formation of more GLUT5. observed in studies Source 1, 6
   ⚖ When the balance tips

   too much — If the capacity of GLUT5 is exceeded, additional fructose remains in the gut lumen; there it holds back water osmotically and passes into the colon.

   too little — If there are few GLUT5 in the gut wall, even a smaller amount of fructose is not fully absorbed; the rest moves on into the colon.

   established physiology · Source 1, 6

2. Into the gut cell → Conversion in gut cell Aldolase B · ATP Part of the fructose is already converted inside the gut cell: fructokinase and aldolase B produce glucose, lactate and other small acids. Fructokinase uses ATP in the process. Source 2↑ supplies The gut cell converts fructose itself via fructokinase and aldolase B and releases glucose and lactate into the blood. In experiments in mice it intercepts small amounts largely in this way before they reach the liver. observed in studies Source 2
   ⚖ When the balance tips

   too much — If more fructose arrives than the gut cell can convert, the excess passes unchanged via the portal vein to the liver and into the colon; this is how it is described in animal models.

   too little — If little fructose arrives in the gut cell, it converts almost all of it itself, and hardly any unchanged fructose reaches the liver.

   observed in studies · Source 2

3. Conversion in gut cell → Via the portal vein GLUT2 What has not been converted leaves the gut cell via GLUT2 and reaches the liver with the portal blood. The liver absorbs a large share of it. Source 1, 2↑ supplies GLUT2 on the side facing the blood releases fructose and the glucose formed in the cell into the portal blood. Both thus reach the liver first, which absorbs a large share of them from the blood. established physiology Source 1, 2
   ⚖ When the balance tips

   too much — If the gut cell releases a lot of fructose via GLUT2, its amount in the portal blood rises, and the liver has to convert more of it.

   too little — If little fructose flows out via GLUT2, the liver receives mainly what the gut cell has already converted to glucose and lactate.

   observed in studies · Source 1, 2

4. Via the portal vein → In the liver Fructokinase · ATP Fructokinase attaches a phosphate, using ATP. Aldolase B splits the molecule into two fragments of three carbons each, which enter glycolysis and gluconeogenesis. With a large supply they also feed fat synthesis. Source 3↕ both, depending on amount Fructokinase works fast, without feedback braking, and uses ATP in the process. The fragments feed glycolysis and gluconeogenesis; with a large supply they also serve as building blocks for fat synthesis in the liver. established physiology Source 3
   ⚖ When the balance tips

   too much — If a lot of fructose arrives rapidly, fructokinase uses ATP faster than it is regenerated; breakdown of the adenine building blocks then yields more uric acid, and ChREBP switches on genes of fat synthesis.

   too little — If little fructose arrives in the liver, the fragments enter glycolysis and gluconeogenesis completely, and fructokinase uses hardly any ATP.

   observed in studies · Source 3

5. Fructose in the colon → Short-chain fatty acids Gut bacteria Colonic bacteria ferment the fructose. This produces short-chain fatty acids such as acetate, propionate and butyrate, which the gut lining absorbs. Butyrate is fuel for colon cells. Source 4↑ supplies The short-chain fatty acids are an energy source for the gut lining; butyrate is the preferred fuel of colon cells. Acetate and propionate reach the liver and other tissues via the blood. established physiology Source 4
   ⚖ When the balance tips

   too much — If many acids form rapidly, the pH in the colon falls; this shifts which bacteria grow and which fermentation products form.

   too little — If few short-chain fatty acids form, less butyrate is available to colon cells as fuel.

   observed in studies · Source 4

6. Fructose in the colon → Hydrogen (H₂) Hydrogenases Fermentation leaves surplus electrons. Bacterial enzymes called hydrogenases pass them to protons, producing hydrogen gas. Only if it is cleared can fermentation continue. Source 4↕ both, depending on amount Hydrogenases dispose of surplus electrons from fermentation as hydrogen. Only if H₂ is cleared – to methane, sulphide, acetate or into the blood – can fermentation continue; if it accumulates, it slows it down. established physiology Source 4
   ⚖ When the balance tips

   too much — If hydrogen builds up in the gut, it inhibits the hydrogenases and slows fermentation; more electrons then go into more reduced products such as lactate or ethanol.

   too little — If little hydrogen forms, or methane-forming and sulphate-reducing microbes use it up quickly, little H₂ remains for blood and breath.

   established physiology · Source 4, 5

7. Hydrogen (H₂) → Methane Archaea · Carbon dioxide In some people the gut hosts archaea that convert hydrogen and carbon dioxide into methane. Less hydrogen and more methane are then found. In doing so they use up hydrogen. Source 4, 5↓ depletes Methane-forming archaea use up four molecules of hydrogen per molecule of methane. They thus withdraw hydrogen from the gut and keep fermentation going; part of the methane is exhaled. established physiology Source 4, 5
   ⚖ When the balance tips

   too much — If many archaea are active, a large part of the hydrogen becomes methane; little H₂ and more methane then appear in the breath.

   too little — If hardly any archaea are present, the hydrogen remains as H₂ or is converted by other microbes to sulphide or acetate.

   established physiology · Source 4, 5

8. Hydrogen (H₂) → H₂ in the blood Part of the hydrogen passes through the gut wall into the blood. Human cells do not produce hydrogen themselves; it comes exclusively from microorganisms. The body does not convert it further. Source 4, 5↑ supplies The blood picks up hydrogen from the gut and carries it to the lungs. The body does not convert it further; it thus reflects how much of it forms in the gut and is not used up by microbes. established physiology Source 4, 5
   ⚖ When the balance tips

   too much — If a lot of hydrogen forms in the gut, more of it passes into the blood, and the amount in the breath rises with it.

   too little — If little hydrogen forms, or microbes already use it up in the gut, little of it reaches the blood.

   established physiology · Source 4, 5

9. H₂ in the blood → H₂ in exhaled air The blood carries the hydrogen to the lungs, where it passes into the breath. This is why exhaled air can be used to follow how much sugar was fermented in the colon. The lungs thus remove it from the body. Source 5↓ depletes The lungs release the hydrogen with the exhaled air and thus remove it from the body. Because it comes only from fermentation, its course after fructose shows when and how much sugar reaches the colon. established physiology Source 5
   ⚖ When the balance tips

   too much — If a lot of fructose reaches the colon, H₂ in the exhaled air rises clearly after the test load.

   too little — If the fructose is absorbed in the small intestine, or the hydrogen is converted to methane by archaea, H₂ in the exhaled air stays low; this is why methane is often measured as well.

   established physiology · Source 5

10. In the liver → DHAP and glyceraldehyde Aldolase B Aldolase B splits fructose-1-phosphate into dihydroxyacetone phosphate (DHAP) and glyceraldehyde. Its blueprint lies in the ALDOB gene; variants that alter the enzyme strongly leave fructose-1-phosphate lying in the cell. Source 8↑ supplies The two three-carbon fragments enter glycolysis, glucose production or fat synthesis. The split also makes the phosphate bound in fructose-1-phosphate usable for the cell again. established physiology Source 8, 3
    ⚖ When the balance tips

    too much — If aldolase B works at full capacity, the fragments flow on quickly; with a large fructose supply they increasingly feed fat synthesis in the liver.

    too little — If aldolase B hardly works, fructose-1-phosphate builds up and traps phosphate; ATP runs short, and glycogen breakdown and glucose production in the liver are inhibited.

    established physiology · Source 8, 3

Further stations

• Fructose in small bowel — from fruit and cane sugar Fructose is a simple sugar. It occurs free in fruit and is part of table sugar, which the enzyme sucrase at the gut wall splits into glucose and fructose. It is the starting material of all following steps. Source 1↑ supplies Fructose is the starting material of the whole pathway. From table sugar it is released only by sucrase; when it arrives together with glucose, the small intestine absorbs it more readily than free fructose alone. established physiology Source 1
  ⚖ When the balance tips

  too much — If more fructose arrives than GLUT5 can absorb, the rest stays in the gut contents; it holds back water osmotically and moves on into the colon.

  too little — If little fructose arrives, the small intestine absorbs it almost completely, and hardly any of it reaches the colon.

  established physiology · Source 1

• Fructose in the colon — portion not absorbed Whatever the small intestine does not absorb moves on into the colon. How much that is depends on the amount and on the rest of the meal. There it draws water into the gut lumen osmotically. Source 1↕ both, depending on amount Fructose that reaches the colon is osmotically active: it draws water into the gut lumen. At the same time it is food for the gut bacteria, which ferment it into fatty acids and gases. established physiology Source 1, 4
  ⚖ When the balance tips

  too much — If a lot of fructose reaches the colon, it draws more water into the gut lumen, and the bacteria form more gas and acids; the gut contents become more liquid and increase in volume.

  too little — If little fructose reaches the colon, the bacteria mainly ferment other undigested carbohydrates.

  established physiology · Source 1, 4

Cofactors in this pathway

• Magnesium — Forms the Mg-ATP complex with ATP that fructokinase uses as its phosphate source; without Mg-ATP turnover stalls Source 7, 3In the ORY catalogue as a laboratory value: Magnesium
• ATP (adenosine triphosphate) — Supplies fructokinase with the phosphate group for fructose 1-phosphate; rapid turnover uses up ATP Source 3
• Niacin (NAD⁺) — NAD⁺ and NADH carry electrons when the fragments are converted to lactate and glucose; NADH becomes NAD⁺ again Source 2, 3In the ORY catalogue as a laboratory value: NAD⁺ (Nicotinamidadenindinukleotid)
• Iron — Metal centre of bacterial hydrogenases, which form hydrogen during fermentation; this lets electrons drain away Source 4

Sources

1. Ferraris RP, Choe JY, Patel CR. Intestinal Absorption of Fructose. Annu Rev Nutr 2018 · PubMed 29751733
2. Jang C, Hui S, Lu W et al. The Small Intestine Converts Dietary Fructose into Glucose and Organic Acids. Cell Metab 2018 · PubMed 29414685
3. Iizuka K. Recent Progress on Fructose Metabolism-Chrebp, Fructolysis, and Polyol Pathway. Nutrients 2023 · PubMed 37049617
4. Carbonero F, Benefiel AC, Gaskins HR. Contributions of the microbial hydrogen economy to colonic homeostasis. Nat Rev Gastroenterol Hepatol 2012 · PubMed 22585131
5. Rezaie A, Buresi M, Lembo A et al. Hydrogen and Methane-Based Breath Testing in Gastrointestinal Disorders: The North American Consensus. Am J Gastroenterol 2017 · PubMed 28323273
6. Douard V, Ferraris RP. Regulation of the fructose transporter GLUT5 in health and disease. Am J Physiol Endocrinol Metab 2008 · PubMed 18398011
7. de Baaij JH, Hoenderop JG, Bindels RJ. Magnesium in man: implications for health and disease. Physiol Rev 2015 · PubMed 25540137
8. Bouteldja N, Timson DJ. The biochemical basis of hereditary fructose intolerance. J Inherit Metab Dis 2010 · PubMed 20162364

Whole pathway: Fructose absorption

Related pathways

• Gut fermentation and breath gases — short-chain fatty acids
• Lactose digestion — short-chain fatty acids
• Gut microbiome — short-chain fatty acids
• Alanine — Magnesium, NAD⁺ (Nicotinamidadenindinukleotid)
• Citric acid cycle — Magnesium, NAD⁺ (Nicotinamidadenindinukleotid)

As of 2026-09-16. Draft written by Claude to schema v2; sources checked in PubMed; expert review pending
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